As is known, portable power tools are usually provided with a two stroke internal combustion engine, which, in addition to being of minor bulk and weight than a four stroke engine, are also constructionally simpler and consequently more economical.
A two stroke internal combustion engine schematically comprises an external casing, which defines a lower engine compartment serving to contain the crank shaft and above at least a cylinder slidingly housing a piston coupled to the crank shaft. The piston defines a combustion chamber with the cylinder head of variable volume, separated by an airtight seal from the engine compartment.
In two stroke engines the engine compartment is provided with an inlet for the fresh air/fuel mixture, while the combustion chamber is provided with an exhaust outlet for the combustion gases. The engine compartment and combustion chamber are reciprocally connected through a transfer duct formed in the engine bodywork and comprising an inlet opening into the engine compartment and an outlet opening into the combustion chamber.
In proximity of the upper stroke limit, the piston closes the combustion chamber exhaust outlet and the transfer duct outlet, leaving open the inlet opening into the engine compartment; while in proximity to the lower stroke limit, the piston leaves open the combustion chamber exhaust outlet and the transfer duct outlet, closing the inlet opening into the engine compartment. Consequently, the operating cycle of a two stroke internal combustion engine is completed in only two operational strokes of the piston in the cylinder, together corresponding to a single complete rotation of the crank shaft.
The first operational stroke starts with the ignition of the air/fuel mixture in the combustion chamber when the piston it located at the upper limit position, and proceeds as the expansion of the gas pushes the piston towards the lower limit position, compressing the fresh air/fuel mixture contained in the engine compartment. During this downward movement the piston opens first the exhaust outlet, such that the combustion gas starts to exit the combustion chamber, and almost simultaneously the piston opens the transfer duct outlet such that the fresh mixture compressed in the engine compartment begins to flow into the combustion chamber until completely filling the chamber while the exhaust gas exits.
During the subsequent rising stroke, the piston compresses the fresh air/fuel mixture contained in the combustion chamber and, before reaching the upper limit position, opens the inlet through which further fresh air/fuel mixture enters the engine compartment as a consequence of the internal depression generated therein.
When the piston reaches the upper limit position, the spark plug produces a spark which ignites the mixture in the combustion chamber and the cycle repeats.
The fresh air/fuel mixture is supplied to the engine compartment through a device that comprises an air intake line, connected to the inlet and opening externally, along which are installed, in series, an air filter, and a carburettor wherein the filtered air arriving from the filter is mixed with the fuel before entering the engine.
During the rising stroke of the piston, when the inlet opens, the air/fuel mixture present in the intake line is accelerated towards the engine and sucked into the engine compartment by the reduced pressure. When the piston closes the inlet during the subsequent downward stroke the air/fuel mixture, previously accelerated, is blocked by the piston and, as a consequence of the inertia of the air/fuel mixture, a pressure wave is generated in the intake line in the opposite direction, from the motor towards the air filter.
As a consequence of this pressure wave a portion of the air and air/fuel mixture already formed in the carburettor can be transported backwards down the intake line and can reach the air filter, where the fuel can damage or produce deposits on the filter screens, resulting in a rapid and drastic reduction in the filtering capacity of the filter screens.
In order to avoid fuel reaching the air filter, it is known practice to install on the intake line, in series between the air filter and the carburettor, an anti-backflow element consisting substantially of an elbow joint in which the air/fluid mixture passing through the intake line is forced to make a marked change in direction.
In this way, the drops of fuel, travelling back down the intake line as a consequence of the pressure wave, cannot pass the elbow joint because the inertial forces of the direction of flow cause the fuel drops to collide against the internal walls of the elbow joint, consequently collecting inside the elbow joint and then subsequently being expelled during the next cycle.
Despite this solution offering good results, the drops of fuel striking the internal walls of the elbow joint can be fractioned by the impact into a plurality of smaller drops, sometimes sufficiently light to remain suspended in the airflow current and consequently to be carried by the pressure wave beyond the anti-backflow element towards the air filter.